Just an FYI in case anybody else ever has this same problem.
As I've mentioned before, for some time I'd had the problem where GPDF would run fine, but would crash anytime that I tried to open a .pdf file with it. A couple days ago I was going around and purging most GNOME-related config files in my home directory in order to try and fix another problem I was having (much bigger problem...). On a hunch afterward, since all of the sudden the GAIM plugin which displays previous conversations in new conversation windows started working again, I grabbed it with apt and tried it again. Worked fine.
Suppose that there was some erroneous config file or other user-specific file that was messing with some apps, though I have no idea how nor what exactly.